It is now common to transport liquefied gases and other materials in tanks positioned within the holds of cargo ships. Particularly, it is well known that liquefied gases, such as LPG, ethylene and LNG, can be transported in tanks permanently attached within the holds of a cargo ship.
The design and construction of liquefied gas carriers is regulated by the International Maritime Organization (IMO) primarily through application of the International Gas Carrier Code (IGC Code). The IGC Code permits a wide range of cargo containment systems. The cylindrical tank system is the most widely employed containment system for liquefied gas carriers having capacities below approx. 22,000 m3. With this system, the cylindrical tanks are supported by two transverse saddles located one near each end of the cylindrical tank. The tank has an internal ring frame at each saddle to help stabilize and distribute the saddle loads into the tank shell. The two saddle system minimizes interaction and resulting stresses between the hull and the tank both of which flex under forces imposed by the ship motions. The diameter and length of such tanks are limited by technical and economic constraints such that the largest single tank known to have been constructed to date has a capacity of about 6,000 m3 and the largest ship capacity is believed to be approximately 12,000 m3.
Larger liquefied gas carriers employ either two smaller diameter tanks fitted side by side or a so called bilobe tank. The bilobe tank consists of two parallel, same diameter horizontal cylinders intersecting each other at about 80% of their diameter. An internal longitudinal bulkhead is fitted where the two “lobes” are joined. As with the cylindrical tank, the bilobe tank is supported by two saddles one near each end. Such tanks can be built to diameters of around 15 m. The largest such tank known to have been built to date is about 7,500 m3 and the largest such liquefied gas carrier employing bilobe tanks has a capacity of around 22,000 m3. Currently, there are studies underway for larger carriers in the range of 40,000 m3.
The interaction between tank and hull due to deformation of each is complex and limits the number of support points to two. The diameter of such tanks is practically limited by the density of the cargo, the design pressure of the tank, saddle spacing, fabrication restrictions and economic factors.
The limitation of two support saddles for each tank results in very large, highly concentrated loads being imposed on the ship's bottom structure. Such “point” loads can exceed 25% of the total loaded ship's displacement (weight in water). These concentrated loads must therefore be distributed throughout the hull structure by way of a complex system of girders and grillage. Such hulls are difficult to fabricate and require more steel than a hull where the cargo load is evenly distributed along the ship's length.
Both of the above tank types are designed as Type C tanks in accordance with the IGC code. Type C tanks are generally designed to comply with land-based pressure vessel codes such as ASME Div. VIII. However, due to the dynamic loads such tanks are subjected to at sea, the IGC Code requires liquefied gas carrier tanks to be designed to increased design pressures, acceleration forces and safety factors as compared to land-based tanks. Therefore Type C tanks are often designed to pressures and loads considerably higher than they will actually experience during their lifetime. This results in large shell material thickness, high tank weight and excessive cost. Since most liquefied gases are carried at atmospheric pressure, the Type C tank is a disadvantage in weight and cost.
Spherical tanks are also used to transport liquefied gases, usually liquefied natural gas at −162° C. Such tanks are designed as Type B tanks of the IGC Code. Type B permits the tanks to be designed to pressures, accelerations and fatigue life as may be actually experienced by the ship during its lifetime. Determining the actual expected design loads is a time consuming and expensive process, but such tanks may be designed with lower material thickness and weight compared to a Type C tank. However, spherical tanks are expensive to fabricate and are generally used only in large liquefied natural gas (LNG) carriers. The largest tanks built to date have a diameter of about 43 m and a volume of around 40,000 m3. In addition to the cost disadvantage, spherical tanks do not utilize the available space in the ship's cargo hold as well as cylindrical tanks and therefore a larger ship must be designed to obtain the same transport capacity.
Independent prismatic tanks are constructed primarily of flat surfaces which are shaped to utilize the ship's form to the greatest possible extent. These tanks may be either Type B tanks or Type A tanks. Type A tanks require the surrounding ship's hull structure to act as a secondary liquid barrier as a protection should the primary liquefied gas tank leak or fail. The surrounding ship's hull structure must therefore be constructed of expensive, low temperature steel which remains tough and crack resistant at the boiling temperature of the liquefied gas (usually LPG, propane or ammonia). Type B prismatic tanks do not need a full secondary barrier and therefore the hull can be built largely of normal ship steel. As with the Type B spherical tank, considerable detailed stress analysis is required to minimize the risk of fatigue or crack propagation. Both tank types have considerable internal support structure similar to the internal hull structure of an oil tanker. Although prismatic tanks have a better volumetric efficiency in the hull than do cylindrical or spherical tanks, they require considerably more material and have limited design pressure.
In case of flooding of the cargo hold by grounding or collision, the cargo tank must be prevented from floating up and breaking through the upper part of the cargo hold. With conventional Type C tanks this is normally accomplished by four large brackets placed on the upper side of the tank in way of the two ring frames. The floatation load is then transmitted through the brackets to the upper hull sides. With spherical tanks, the tank equator is welded to the ship's structure via a so called skirt and therefore the support structure also holds the tank against floatation. With prismatic tanks the hold down is accomplished by brackets located on the upper sides of the tanks and attached to the sides of the ship in numerous locations.